The short answer is NO! If you suspect acetaminophen overdose then you should properly identify a time or range for a 4 hr level and obtain the specimen at that time.

However – “Only very low to undetectable acetaminophen concentrations prior to 4 hours reliably excluded a subsequent concentration over the treatment line.” We can use this to our advantage for patients who we do NOT suspect to have an acetaminophen overdose!

In this UK study, none of the 136 patients who denied taking acetaminophen had a detectable level at 4 hours.

So although you can lower the prior probability of acetaminophen poisoning when the patient denies ingestion, you cannot completely exclude the diagnosis without an acetaminophen level. However, you can save a few hours of ED time – here’s how:

When you do NOT suspect an acetaminophen overdose (prior probability is close to zero!) , an undetectable level before 4 hrs (at 2 hours and even as soon as 1 hour) post ingestion will reliably exclude acetaminophen toxicity (likelihood ratio of zero!). If you draw a < 4hr level and it shows detectable acetaminophen – it must be repeated at 4 hours to determine if it needs treatment.

If you DO suspect an acetaminophen overdose, obtain only a 4 hour level to guide NAC therapy.

Hopefully this will help with a more rapid throughput of patients with self poisoning who need to be medically cleared for further psychiatric evaluation. Of course, this represents one opinion based on a review of the literature. A prospective trial would be needed to prove this conclusively.